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As if a world dominated by hungry, house-sized lizards weren't sufficiently exciting, scientists have added another set piece to our image of the Cretaceous: raging wildfires.
The Cretaceous period, which ended about 65 million years ago with the extinction of the dinosaurs, was hot. That's thanks to volcanos that pumped carbon dioxide into the atmosphere and created a greenhouse effect. Researchers from London and Chicago now say it was also a "high-fire" world. Frequent blazes may have kept animals on the run, created some of the fossil beds we study today, and helped determine which plant species survived into the next era.
Led by graduate student Sarah Brown from the Royal Holloway University of London, the researchers tracked the appearance of charcoal in ancient sediments. Like a set of sooty footprints right through the fossil record, the charcoal evidence showed when and where fires had occurred.
The team saw that wildfires had increased during the Cretaceous period. These fires were probably sparked by lightning, and their flames were fanned by the high concentration of oxygen in the ancient atmosphere. Today, oxygen makes up about 21% of our air. But during the Cretaceous, it may have risen as high as 25% or more.
This high oxygen content, the authors say, would have allowed plants to burn without being bone dry. A spark in a green forest, instead of dying out as it would today, might become a full-blown fire.
Brown and her coauthors did not find any evidence that these fires contributed to killing off the dinosaurs. But they note that after a fire burns through a piece land, erosion is likely. There may be rapid flooding or mudslides. In the Cretaceous, these events might have trapped and killed dinosaurs and other animal life--and helped preserve their bones.
The authors point to certain fossil beds that lie in floodplains and contain charcoal, as well as plant and animal remains. These could be sites where wildfires triggered flooding, conveniently sweeping lots of informative fossils into one place for future scientists to find.
Charred plant remains in these fossil beds provide another clue about the effect of fire. As the Cretaceous went on, the types of plants being fossilized gradually changed. Flowering plants, called angiosperms, became more and more common. Gymnosperms--the more ancient, flowerless species such as cone-bearing trees, cycads, and ginkgos--faded into the background.
A charred flower fossil from the Late Cretaceous.
Frequent fires may have given an added edge to the angiosperms. The new types of plumbing these plants had invented let them grow faster and more efficiently. Rather than trees, the flowering plants growing during the Cretaceous seem to have been weedy and shrubby types. After a fire, they could regrow faster than the gymnosperms. And their new growth provided fresh fuel for wildfires, creating a cycle that encouraged the growth of flowering plants and left older models in the dust.
Though fire didn't do in the dinosaurs, then, it may have helped set the stage for the dominant plants of the modern age. (As if we needed any more drama.)
Brown, S., Scott, A., Glasspool, I., & Collinson, M. (2012). Cretaceous wildfires and their impact on the Earth system Cretaceous Research DOI: 10.1016/j.cretres.2012.02.008
Images: Gorgosaurus from Nobu Tamura/Wikimedia Commons; flower fossil from Brown et al.
Just thought I would mention as an interesting aside that in at least some modern-day forests, conifers actually love fire, while the angiosperms hate it. Southern pine growers (including pretty much my mom's whole family in Alabama) use this to their/our advantage by burning up the woods every couple of years (very carefully). The pines (especially longleaf) can survive fire beyond the age of 5 or so, whereas a good fire will definitely kill younger oaks and other hardwoods, and even has a pretty good chance of killing off a fully grown non-pine if it gets hot enough or the fire is repeated soon enough. Although we now do it in a controlled way, it's been going on forever either via American Indian humans or random lightning strikes. There's all sorts of cool related traits: for example, pine needles burn really easily and encourage fire, while oak leaves and hardwood litter will pretty much go out unless it's SUPER dry. I wouldn't necessarily want to make the case that those things evolved in that way because of the fire stuff, but maybe.
ReplyDeleteAnyway, it had never occurred to me that this wasn't true of all gymnosperm/angiosperm competitive scenarios, so I was interested to see the opposite case made in this article. I guess the angiosperms in question in the Cretaceous are actually (as you mentioned) more like shrubs, which are on a totally different time cycle than the 30- or 40-year tree growing cycle I'm used to.
Cool! Yes, gymnosperms have some great adaptations for fire. The article mentions that tree bark, fire-resistant spore coverings on ferns, and pinecones that need heat to open all may have evolved during the Cretaceous. Maybe in this period the shrubby angiosperms outcompeted the slow-growing gymnosperms, but the gymnosperms that survived adapted to fire.
ReplyDeleteNorman, I was just thinking the same thing. I wonder if conifer adaptations to fire like very thick bark and closed (serotinous) cones actually evolved later, or if they existed back then. My guess is they probably did exist at least to some extent.
ReplyDeleteCertainly any change to fire frequency or intensity can cause big changes in plant communities. There are big issues with invasive plants altering fire regimes and proliferating explosively in the southwestern US.